Clinical meaning
Tetralogy of Fallot (TOF) is the most common cyanotic congenital heart defect, accounting for 7-10% of all congenital heart disease. It results from anterior and cephalad deviation of the infundibular (conal) septum during embryonic cardiac development, producing four anatomical features: (1) Right ventricular outflow tract obstruction (RVOTO) — the most critical hemodynamic feature, ranging from mild infundibular stenosis to complete pulmonary atresia; may involve infundibular (subvalvular), valvular, supravalvular, or branch pulmonary artery stenosis; (2) Ventricular septal defect (VSD) — large, nonrestrictive, malalignment type, allowing equalization of RV and LV pressures; (3) Overriding aorta — the aortic root is displaced rightward, overriding the VSD and receiving blood from both ventricles; (4) Right ventricular hypertrophy — secondary to RV pressure overload from RVOTO. The degree of RVOTO determines the severity of cyanosis: mild obstruction → minimal right-to-left shunting (acyanotic or 'pink tet'); severe obstruction → significant right-to-left shunting across the VSD → cyanosis. 'Tet spells' (hypercyanotic episodes) are paroxysmal events where RVOTO acutely worsens (infundibular spasm, decreased systemic vascular resistance, or increased venous return): increased right-to-left shunting → profound cyanosis → acidosis...